Unlike conventional organic pigments, which are usually precipitated single components, fluorescent pigments are solid state solutions of common fluorescent dyes in polymeric resins or matrices. Today's fluorescent pigment industry relies heavily on matrices that are formed by melt condensation of melamine, formaldehyde, and toluenesulfonamide. In order to manufacture products that possess the requisite resistance to many common organic solvents utilized in the paint and printing ink industries, the formaldehyde based products are formulated to yield thermosetting materials. These products are typically manufactured in a two-stage process, in which the resinous B-stage is converted to the C-stage by baking with convection ovens or infra-red panels. With some difficulty, the high molecular weight thermoset materials are reduced in particle size by conventional grinding methods.
Because two steps plus grinding are involved, a lot of waste occurs in the production of the conventional thermoset pigments.
These pigments have been used in a variety of solvents; however, they have a tendency to swell in many organic solvents. Swelling of the polymer matrix increases the viscosity of the solvent vehicle used in applying the pigments to various substrates. Viscosity increases and concomitant polymer swelling result in a shortened product shelf life, and in some instances, undesirable performance.
It is an object of the invention to provide a fluorescent pigment which affords good viscosity control when placed in a solvent. That is, the viscosity level of the solvent containing the pigment is stable with aging because the pigment resists swelling and dissolution.
Today, free formaldehyde is an environmental concern. It is difficult to totally remove unreacted formaldehyde from existing thermoset pigments. For this reason, it is an object of this invention to provide a product which does not require formaldehyde.